"Gibbs and [Lara] have some past history that goes back a long, long way," Brennan said. "And quite clearly, Gibbs is not very keen to actually fly to Los Angeles and work with her - particularly when she's given the lead on the case."

Louise Lombard, right, will play a character named Lara on the NCIS spin-off. She and Gibbs have an interesting back story.

So, is this baggage romantic in nature? Brennan won't say. But...

"It is resolved over the two-part episode. The audience will find out exactly what it is, and I can guarantee no one is going to guess what the connection is."